Johnson lithium oxygen electrochemical engine

ABSTRACT

A rechargeable lithium air battery is provided. The battery contains a ceramic separator forming an anode chamber, a molten lithium anode contained in the anode chamber, an air cathode, and a non-aqueous electrolyte. The cathode has a temperature gradient comprising a low temperature region and a high temperature region, and the temperature gradient provides a flow system for reaction product produced by the battery.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/281,875, filed Jan. 22, 2016, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The need for high performance and reliable energy storage in the modernsociety is well documented. Lithium batteries represent a veryattractive solution to these energy needs due to their superior energydensity and high performance. However, available Li-ion storagematerials limit the specific energy of conventional Li-ion batteries.While lithium has one of the highest specific capacities of any anode(3861 mAh/g), typical cathode materials such as MnO₂, V₂O₅, LiCoO₂ and(CF)n have specific capacities less than 200 mAh/g.

Recently, lithium/oxygen (Li/O₂) or lithium air batteries have beensuggested as a means for avoiding the limitations of today's lithium ioncells. In these batteries, lithium metal anodes are used to maximizeanode capacity and the cathode capacity of Li air batteries is maximizedby not storing the cathode active material in the battery. Instead,ambient O₂ is reduced on a catalytic air electrode to form O₂ ²⁻, whereit reacts with Li⁺ions conducted from the anode. Aqueous lithium airbatteries have been found to suffer from corrosion of the Li anode bywater and suffer from less than optimum capacity because of the excesswater required for effective operation.

Abraham and Jiang (J. Electrochem. Soc., 1996, 143 (1), 1-5) reported anon-aqueous Li/O₂ battery with an open circuit voltage close to 3 V, anoperating voltage of 2.0 to 2.8 V, good coulomb efficiency, and somere-chargeability, but with severe capacity fade, limiting the lifetimeto only a few cycles. Further, in non-aqueous cells, the electrolyte hasto wet the lithium oxygen reaction product in order for it to beelectrolyzed during recharge. It has been found that the limitedsolubility of the reaction product in available organic electrolytesnecessitates the use of excess amounts of electrolyte to adequately wetthe extremely high surface area nanoscale discharge deposits produced inthe cathode. Thus, the required excess electrolyte significantlydecreases high energy density that would otherwise be available inlithium oxygen cells.

Operation of Li/O₂ cells depends on the diffusion of oxygen into the aircathode. Oxygen absorption is a function of the electrolyte's Bunsencoefficient (α), electrolyte conductivity (σ), and viscosity (η). It isknown that as the solvent's viscosity increases, there are decreases inlithium reaction capacity and Bunsen coefficients. Additionally, theelectrolyte has an even more direct effect on overall cell capacity asthe ability to dissolve reaction product is crucial. This problem haspersisted in one form or another in known batteries.

Indeed, high rates of capacity fade remain a problem for non-aqueousrechargeable lithium air batteries and have represented a significantbarrier to their commercialization. The high fade is attributedprimarily to parasitic reactions occurring between the electrolyte andthe mossy lithium powder and dendrites formed at the anode-electrolyteinterface during cell recharge, as well as the passivation reactionsbetween the electrolyte and the LiO₂ radical which occurs as anintermediate step in reducing Li₂O₂ during recharge.

During recharge, lithium ions are conducted across the electrolyteseparator with lithium being plated at the anode. The recharge processcan be complicated by the formation of low density lithium dendrites andlithium powder as opposed to a dense lithium metal film. In addition topassivation reactions with the electrolyte, the mossy lithium formedduring recharge can be oxidized in the presence of oxygen into mossylithium oxide. A thick layer of lithium oxide and/or electrolytepassivation reaction product on the anode can increase the impedance ofthe cell and thereby lower performance. Formation of mossy lithium withcycling can also result in large amounts of lithium being disconnectedwithin the cell and thereby being rendered ineffective.

Lithium dendrites can penetrate the separator, resulting in internalshort circuits within the cell. Repeated cycling causes the electrolyteto break down, in addition to reducing the oxygen passivation materialcoated on the anode surface. This results in the formation of a layercomposed of mossy lithium, lithium-oxide and lithium-electrolytereaction products at the metal anode's surface which drives up cellimpedance and consumes the electrolyte, bringing about cell dry out.

Attempts to use active (non lithium metal) anodes to eliminate dendriticlithium plating have not been successful because of the similarities inthe structure of the anode and cathode. In such lithium air “ion”batteries, both the anode and cathode contain carbon or anotherelectronic conductor as a medium for providing electronic continuity.Carbon black in the cathode provides electronic continuity and reactionsites for lithium oxide formation. To form an active anode, graphiticcarbon is included in the anode for intercalation of lithium and carbonblack is included for electronic continuity. Unfortunately, the use ofgraphite and carbon black in the anode can also provide reaction sitesfor lithium oxide formation. At a reaction potential of approximately 3volts relative to the low voltage of lithium intercalation intographite, oxygen reactions would dominate in the anode as well as in thecathode. Applying existing lithium ion battery construction techniquesto lithium oxygen cells would allow oxygen to diffuse throughout allelements of the cell structure. With lithium/oxygen reactions occurringin both the anode and cathode, creation of a voltage potentialdifferential between the two is difficult. An equal oxidation reactionpotential would exist within the two electrodes, resulting in novoltage.

As a solution to the problem of dendritic lithium plating anduncontrolled oxygen diffusion, known aqueous and non-aqueous lithium airbatteries have included a barrier electrolyte separator, typically aceramic material, to protect the lithium anode and provide a hardsurface onto which lithium can be plated during recharge. However,formation of a reliable, cost effective barrier has been difficult. Alithium air cell employing a protective solid state lithium ionconductive barrier as a separator to protect lithium in a lithium aircell is described in U.S. Pat. No. 7,691,536 of Johnson. Thin filmbarriers have limited effectiveness in withstanding the mechanicalstress associated with stripping and plating lithium at the anode or theswelling and contraction of the cathode during cycling. Moreover, thicklithium ion conductive ceramic plates, while offering excellentprotective barrier properties, are extremely difficult to fabricate, addsignificant mass to the cell, and are rather expensive to make.

As it relates to the cathode, the dramatic decrease in cell capacity asthe discharge rate is increased is attributed to the accumulation ofreaction product in the cathode. At high discharge rate, oxygen enteringthe cathode at its surface does not have an opportunity to diffuse orotherwise transition to reaction sites deeper within the cathode. Thedischarge reactions occur at the cathode surface, resulting in theformation of a reaction product crust that seals the surface of thecathode and prevents additional oxygen from entering. Starved of oxygen,the discharge process cannot be sustained.

Another significant challenge with lithium air cells has beenelectrolyte stability within the cathode. The primary discharge productin lithium oxygen cells is Li₂O₂. During recharge, the resulting lithiumoxygen radical, LiO₂, an intermediate product which occurs whileelectrolyzing Li₂O₂, aggressively attacks and decomposes the electrolytewithin the cathode, causing it to lose its effectiveness.

High temperature molten salts have been suggested as an alternative toorganic electrolytes in non-aqueous lithium-air cells. U.S. Pat. No.4,803,134 of Sammells describes a high lithium-oxygen secondary cell inwhich a ceramic oxygen ion conductor is employed. The cell includes alithium-containing negative electrode in contact with a lithium ionconducting molten salt electrolyte, LiF—LiCl—Li₂O, separated from thepositive electrode by the oxygen ion conducting solid electrolyte. Theion conductivity limitations of available solid oxide electrolytesrequire that such a cell be operated in the 700° C. range or higher inorder to have reasonable charge/discharge cycle rates. The geometry ofthe cell is such that the discharge reaction product accumulates withinthe molten salt between the anode and the solid oxide electrolyte. Therequired space is an additional source of impedance within the cell.

TABLE 1 Physical properties of Molten Nitrate Electrolytes Melt Temp κ(S/cm) System Mol % ° C. @570K at Mol % LiNO₃—KNO₃ 42-58 124 0.687 50.12mol % LiNO₃ LiNO₃—RbNO₃ 30-70 148 0.539   50 mol % RbNO₃ NaNO₃—RbNO₃44-56 178 0.519   50 mol % RbNO₃ LiNO₃—NaNO₃ 56-44 187 0.985 49.96 mol %NaNO₃ NaNO₃—KNO₃ 46-54 222 0.66 50.31 mol % NaNO₃ KNO₃—RbNO₃ 30-70 2900.394   70 mol % RbNO₃

Molten nitrates also offer a viable solution and the physical propertiesof molten nitrate electrolytes are summarized in Table 1 (taken fromLithium Batteries Using Molten Nitrate Electrolytes by Melvin H. Miles;Research Department (Code 4T4220D); Naval Air Warfare Center WeaponsDivision; China Luke, Calif. 93555-61000).

The electrochemical oxidation of the molten LiNO₃ occurs at about 1.1 Vvs. Ag+/Ag or 4.5 V vs. Li+/Li. The electrochemical reduction of LiNO₃occurs at about −0.9V vs. Ag+/Ag, and thus these two reactions define a2.0V electrochemical stability region for molten LiNO₃ at 300° C. andare defined as follows:

LiNO₃→Li⁺+NO₂+½O₂ +e ⁻  (Equation 1)

LiNO₃+2e ⁻→LiNO₂+O⁻⁻  (Equation 2)

This work with molten nitrates was not performed with lithium air cellsin mind; however, the effective operating voltage window for theelectrolyte is suitable for such an application. As indicated by thereaction potential line in FIG. 1, applying a recharge voltage of 4.5Vreferenced to the lithium anode can cause lithium nitrate to decomposeto lithium nitrite, releasing oxygen. On the other hand, lithium canreduce LiNO₃ to Li₂O and LiNO₂. This reaction occurs when the LiNO₃voltage drops below 2.5V relative to lithium. As long as there isdissolved oxygen in the electrolyte, the reaction kinetics will favorthe lithium oxygen reactions over LiNO₃ reduction. Oxide ions arereadily converted to peroxide (O₂ ²⁻) and aggressive superoxide (O₂ ⁻)ions in NaNO₃ and KNO₃ melts (M. H. Miles et al., J. Electrochem. Soc.,127,1761 (1980)).

A need remains for a lithium air cell which overcomes problemsassociated with those of the prior art.

BRIEF SUMMARY OF THE INVENTION

A rechargeable lithium air battery comprises a ceramic separator formingan anode chamber, a molten lithium anode contained in the anode chamber,an air cathode, and a non-aqueous electrolyte, wherein the cathode has atemperature gradient comprising a low temperature region and a hightemperature region, and wherein the temperature gradient provides a flowsystem for reaction product produced by the battery.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a diagram depicting electrochemical reaction potentials inmolten lithium nitrate at 300° C.;

FIG. 2 is a schematic of a battery cell according to one embodiment ofthe present invention;

FIG. 3 is a schematic of a battery cell according to another embodimentof the present invention in discharge;

FIG. 4 is a schematic of the battery cell of FIG. 3 in recharge;

FIG. 5 is a schematic of a high performance battery cell according to afurther embodiment of the invention in discharge;

FIG. 6 is a schematic of a high performance battery cell of FIG. 5 inrecharge;

FIG. 7 is a schematic of a battery cell according to a furtherembodiment of the invention; and

FIG. 8 is an Arrhenius plot showing lithium ion conductivities ofseveral solid ceramic electrolytes.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates generally to energy storage, and moreparticularly to a lithium air electrochemical cell. For the purposes ofthis disclosure, the terms lithium air cell, lithium air electrochemicalengine and lithium oxygen battery are used interchangeably.

The present invention provides a rechargeable lithium air cell having ahigh rate of cell charge/discharge with limited capacity fade, highenergy density, high power density, and the ability to operate on oxygenfrom ambient air. As such, it removes significant barriers that haveprevented the commercialization of lithium air cells. For example, theformation of mossy lithium powder and dendrites at the anode-electrolyteinterface during cell recharge are eliminated by the use of moltenlithium supplied as a flow reactant to the anode side of a stable solidstate ceramic electrolyte. The battery according to the invention alsoincludes a flow system for removing reaction product from the cathode.

The reactions of lithium with oxygen are as follows:

2Li+O₂→Li₂O₂ E_(o)=3.10 V

4Li+O₂→2Li₂O E_(o)=2.91V

To avoid problems associated with past approaches to lithium air cells,a lithium air cell according to the invention may be operated at a widerange of temperatures in the range of 20° C. to 700° C., which includeelevated temperatures, such as the preferred temperatures of about 200°C. to 450° C., more preferably about 200° to about 250° C. The solventin the electrolyte may be selected based on the preferred operatingtemperature of the specific battery. Operation at elevated temperatureenables faster kinetics for higher power density, thus eliminating amajor issue associated with lithium air technology. Further, operationat elevated temperature also allows for the use of high temperatureorganic electrolytes and inorganic, molten salt electrolyte solutionsthat have high electrochemical stability, thus avoiding another of themajor problems that has plagued conventional approaches to lithium aircells. Selected inorganic molten salts have good solubility oflithium/oxygen reaction products, thus allowing better control of cellkinetics.

The rechargeable air battery according to the invention contains aceramic separator which forms an anode chamber, a molten lithium anodecontained in the anode chamber, an air cathode, and a non-aqueouselectrolyte. Each of these components will be described in more detailbelow.

The cell further comprises a flow system which is provided by atemperature gradient across the cathode. More specifically, the cathodehas two temperature regions: a high temperature region (preferablylocated near the anode, where the reaction takes place) and a lowtemperature region which is located further away from the anode. As theelectrolyte circulates through the cell during discharge, the reactionproduct produced by the battery migrates from the high temperatureregion to the low temperature region.

The anode chamber is preferably formed by a sealed ceramic enclosurethat is lithium ion conductive and which functions as the separator forthe battery. Preferably, the ceramic material is stable in contact withlithium metal and protects the anode from ambient oxygen and moisture.Preferred materials include lithium ion conducting glasses such aslithium beta alumina, lithium phosphate glass, lithium lanthanumzirconium oxide (LLZO), Al₂O₃:Li₇La₃Zr₂O₁₂, lithium aluminum germaniumphosphate (LAGP), and lithium aluminum titanium phosphate (LATP). In apreferred embodiment, the anode chamber is maintained at about 20° C. toabout 200° C., more preferably at about 175° C. to about 200° C., mostpreferably about 175° C. to about 195° C.

The anode comprises metallic lithium in a molten state; lithium has amelting point of about 180° C. The benefit of the molten lithium anodeis that it limits undesirable dendrite growth in the cell.

The non-aqueous electrolyte is chosen for stability in contact withlithium. Thus, a breach in the ceramic enclosure will not result inrapid reactions, particularly because air ingress into the cell will becontrolled. Preferred electrolytes include molten inorganic salts, forexample, alkali nitrates such as lithium and sodium nitrate, alkalichlorides and bromides such as lithium, potassium and sodium chloridesand bromides, alkali carbonates such as sodium and lithium carbonates,as well as sodium nitrate-potassium nitrate (NaNO₃—KNO₃) eutecticmixtures and silane and siloxane-based compounds including, for example,hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane,decamethylcyclopentasiloxane, and dodecamethylhexatetrasiloxane with orwithout polyethylene oxide groups.

The inorganic salt, silane, or siloxane in the electrolyte is present ina solvent. The solvent is not limited, and may be selected based on thepreferred operating temperature of the battery. A preferred solvent isLiCl—KCl eutectic, which works at a temperature of 350° C. to 450° C.The temperature of the electrolyte may be controlled with a heater andis preferably about 200° C. to 450° C.

The air cathode or positive electrode is porous so that oxygen canpenetrate through the pores and form lithium peroxide as the reactionproduct; electrolyte also flows through the porous cathode. The cathodeis preferably formed from a porous ceramic material which is lithiumconductive and which is infiltrated or impregnated with a metal nitratesuch as silver nitrate or a carbon material such as carbon fibers,carbon black, or carbon foam. Preferred porous ceramic materials includeLLZO, LAGP, LATP, and lithium oxyanions such as lithium carbonate; mostpreferred is LLZO. In another preferred embodiment, the cathode containsa carbon material, a heat resistance polymer binder such as polyimide,and a metal oxide catalyst. An exemplary cathode material of this typecontains about 60% by weight vapor grown carbon fibers, about 30%polyimide binder, and about 10% manganese dioxide. The cathode may alsobe constructed of electrically conductive sintered metal oxide powder,sintered metal nitride, carbon, or sintered silicon carbide.

As a preferred example, porous lithium lanthanum zirconium oxide (LLZO)ceramic substrates are prepared by pressing 10-15 grams of LLZO powderinto a disc at 1000 psi. The disc is densified by placing in a furnaceat 1000° C. for a period of 1 hour. The disc is then impregnated with ametal nitrate such as silver nitrate to form the cathode.

A thermodynamic process is employed to remove and supply electrolyte tocathode reaction sites. In its basic configuration, a temperaturegradient is maintained across the structure of the cathode wetted by theelectrolyte. The active charge/discharge reaction region of the cellforms the higher temperature region of the gradient. As a result of thetemperature gradient, during discharge, reaction product accumulatedwithin the electrolyte at the higher temperature region migrates to thelower temperature region where it precipitates/solidifies. Theconfiguration of the cell is such that reaction product can accumulatewithin the lower temperature region physically away from the highertemperature reaction region of the cell. Accumulation of reactionproduct in the lower temperature region prevents it from significantlyaffecting the charge/discharge cell kinetics occurring in the highertemperature cathode reaction region. Ultimately, the cooled and settledreaction product will become re-dissolved in the electrolyte. This flowsystem is a key attribute of the inventive batteries.

In an alternative embodiment, the cell contains a pump to circulate theelectrolyte across the temperature gradient. Such a cell contains amolten or another appropriate electrolyte reservoir and a temperaturecontrol system for controlling the relative temperatures of the cathodeand the reservoir. Further, a heating element is employed forelectrolyte temperature control. The pump system cycles electrolytebetween the cathode and the electrolyte reservoir, which are adjacent toand in fluid communication with each other. Operation is such thatduring discharge, the cathode is maintained at a temperature that iselevated above that of the electrolyte reservoir. Reaction productdissolved in the electrolyte at high temperature in the cathode iscarried to the electrolyte reservoir where it precipitates due to thelower temperature therein. In contrast, during charge, heat is suppliedto the reservoir to maintain solubility of reaction product into theelectrolyte. During charge, the electrolyte carries dissolved reactionproduct from the reservoir to the cathode, where it is electrolyzed.Oxygen is released and lithium ions are conducted through the ceramicseparator such that lithium metal is plated at the anode. Electrolytedepleted of reaction product circulates back to the reservoir where itdissolves and carries more reaction product to the cathode as the chargeprocess continues. The configuration is such that the reaction productis temporarily stored as a solid in the electrolyte reservoir as opposedto the cathode. Operation in this manner enables the cathode to bemaintained in an optimum configuration for maximum charge and dischargeperformance.

FIG. 2 is a schematic drawing of a molten lithium electrochemical cellaccording to an embodiment of the invention. The cell is cylindrical inshape with fins running lengthwise along the cylinder and radiatingoutward away from the core of the cell. The basic structure is supportedby hollow solid electrolyte cylinder (anode chamber) 2 which extends thelength of the cell and functions as the cell separator. Molten lithiummetal 14 is contained within reservoir 18 at the top of the cell andinside annular cavity 4 such that molten lithium is free to flow downfrom reservoir 18 into annular cavity 4. The top level of the moltenanode 16 is not expected to totally fill the headspace 20 of the cell.Electrical heater element 6 runs the length of the cell and ispositioned to maintain the lithium in a molten state. Heater 6 is partof the core structure that forms annular cavity 4 between the heater andthe inner wall of the solid electrolyte 2 where molten lithium 14 iscontained. Lithium 14 serves as the anode of the cell. Fined cathodecylinder 8 is positioned over the outer surface of electrolyte cylinder2. The core of the fin is shown by 9. Cathode 8 is a porous structurecontaining liquid electrolyte which, due to its finned structure, isconfigured to have a wicking effect to maintain distribution ofelectrolyte therein. The reaction in the cell occurs at the interfacewhere the cathode touches the separator, which is the hotter (hightemperature) region of the cathode. The reaction product will not settlein this hot portion of the cathode, but rather on the colder side of thecathode (low temperature region). This allows for deeper cathode access.The cell preferably operates at 250° C. to 700° C. such that theeutectic salt mixture or other electrolyte is maintained in a moltenstate. Fins 10 extend into the surrounding air to facilitate heattransfer to the air such that heat supplied to the core induces atemperature gradient radially outward that is maintained between tips 12of the fins 10 and the molten lithium at the core of the cell.

Dissolved reaction product 11 generated during discharge willpreferentially precipitate in the lower temperature regions of the finsas opposed to the warmer core region. Molten electrolyte reservoir 1contains excess electrolyte 3 and electrolyte that has been displaced byreaction product as it is produced and deposited within fins 10.Reservoir 1 may be maintained at a temperature that is lower than thecore of the cell such that the reaction product preferentiallyprecipitates therein as well. The temperature of the reservoir iscontrolled by heater element 5. During recharge, reaction productre-dissolves into the molten salt electrolyte to maintain concentrationequilibrium as product is electrolyzed and lithium is re-plated at theanode. Heater 5 is used during recharge to heat the electrolyte toredissolve reaction product. The heat source for core 6 of the cell isnot shown but would maintain temperature for operation during bothcharge and discharge.

Reservoir 18 supplies lithium 14 to annular cavity 4 so that the cavitydoes not become depleted as the lithium is consumed during discharge.Similarly, as lithium is reduced into the annular section duringrecharge, lithium is resupplied and accumulated in the reservoir.

FIGS. 3 and 4 show expanded views of radial plane cross section 26 ofthe cell in FIG. 2 and illustrate the operation of the cell. These Figs.show heater/spacer 6 including heater element 7, finned cathode 8,annular lithium cavity 4, solid electrolyte cylinder 2 and moltenlithium anode 14. Referring to FIG. 3, oxygen 47 dissolves into themolten salt electrolyte from the cell's environment. During discharge,lithium 44 is oxidized and conducted through electrolyte separator 2into the molten salt contained within cathode 8, giving rise to electriccurrent flow 45 through load 40 to cathode 8. The electrons 43 oxidizemolecular oxygen that is dissolved in the molten salt electrolyte,producing oxygen ions 46 to complete the reaction, with the resultingreaction product being either lithium peroxide (Li₂O₂ as 2Li⁺and O₂ ⁻⁻)and/or lithium oxide (Li₂O as 2Li⁺and O⁻⁻) ions suspended in the moltensalt electrolyte solution. The two lithium ions 42 are anticipated to beindividually dispersed within the electrolyte. The illustration is notintended to convey a diatomic pair bonded to each other. When the moltensalt becomes saturated with reaction product, lithium peroxide 48 and/orlithium oxide begins to precipitate out of solution.

Heater element 7 located in the center region of the cell maintains thelithium anode and the electrolyte salt contained in the cathode in amolten state. Because of its location and because of the loss of heatfrom the cathode fins to the air surrounding the cell, a decreasingtemperature exists between the core of the cell 6 and fin tips 12. Themolar equilibrium of dissolved lithium/oxygen reaction product in themolten salt will be lower at the lower temperature fin tips 12 than atthe high temperature cathode material 45 that is closest to the core ofthe cell. As such, reaction product 48 will tend to precipitate out ofsolution in the region of fin tips 12, resulting in a buildup ofreaction product 41 in that location. Although reaction kinetics willfavor the high temperature region, creation of reaction product in hightemperature region 14 will cause over saturation and precipitation ofreaction product in lower temperature fin tip region 12. Migration tofin tips 12 will occur because the molar concentration of reactionproduct in the salt is continuous between the two regions. The saltlevel will naturally be uniformly distributed, limited only by masstransport rate across the concentration gradients of the dissolvedproduct within the molten salt. Further production of reaction productsin the solution in the higher temperature regions will causeprecipitation of reaction product in the lower temperature region sincethe increase would cause over saturation in the low temperature region.

Having the reaction product accumulate in the fin tip regions of thecell is important because precipitation in this region has only verylimited adverse impacts on operation of the cell. The invention thusavoids over accumulation of reaction product in the active region of thecell which could cause a reduction of ionic conductivity and could blockaccess and diffusion of oxygen to reaction sites.

FIG. 4 depicts recharge operation of the cell. For recharge, powersource 50 is connected in the circuit in place of the load. Dissolvedlithium/oxygen reaction product 52, 54, 56 is electrolyzed as electrons53 are stripped by the power source and coupled to the anode side of thecell. During the process, molecular oxygen 57 is released to theenvironment and lithium ions 54 are conducted through the solid stateseparator 2 to the anode side of the cell where electrons 53 reduce itto lithium metal.

As reaction product 58 is consumed from the molten salt electrolytesolution, its molar concentration level in the electrolyte eutectictends lower, thus allowing additional reaction product precipitant 41 todissolve into the electrolyte. The re-dissolved reaction productnaturally migrates toward the core region of the cell due to theconcentration gradient created as reaction product in the core region isremoved by the recharge process. Continuous dissolving of reactionproduct 41 maintains a molar equilibrium concentration level of thereaction product in the electrolyte in fin tip region 12 until all ofdischarge reaction product 41 is re-dissolved and electrolyzed, wherebythe cell will be fully charged.

FIG. 5 is a schematic diagram of a high performance lithium oxygen orlithium air cell according to a further embodiment of the invention.Lithium reservoir 62 contains molten lithium 64 at a preferredtemperature of 350° C. A portion 72 of lithium reservoir 62 extends intoreactor chamber 68 where separator 71 interfaces with the contents ofchamber 68. Reservoir 62 optionally includes ullage pressurized gas 66to ensure flow of molten lithium into contact with solid stateelectrolyte separator 71. Reservoir 62 maintains the supply 101 oflithium to separator 71 as the cell is discharged. Separator 71 is asolid lithium ion conductive material and may be lithium beta alumina orlithium lanthanum zirconium oxide (LLZO). It is preferably a solidceramic and/or a glass electrolyte. Cathode 98 and embedded currentcollector 74 are coupled to the surface of separator 71 on the externalside of reservoir 62. Cathode 98 includes lithium/oxygen reaction sitesfor charge and discharge of the cell. Current collector 74 is connectedto positive terminal 69 which allows electrons 81 to travel. Power isapplied to terminals 82. Reactor chamber 68 contains molten electrolyte78. Pump 75 supplies electrolyte solution 78 through supply tube 76 tonozzle 80. Nozzle 80, tube 85 and port 87 comprise a jet pump wherebyfluid supplied by pump 75 creates a low pressure region that draws air84 into port 87 such that it flows through conduit 86 to port 87. Thefluid injection process creates a turbulent mixture region of air andmolten electrolyte. It produces a washing effect as the resulting spray104 exits the jet pump and impinges on cathode 98. This process createsan electrochemical potential between the lithium inside reservoir 62 onone side of electrolyte 71 (electrode terminal 70) and the oxygendissolved and dispersed within electrolyte/air mixture washing throughcathode 98 on the other side.

Operation of the cell is such that molten salt electrolyte 102 washingthrough cathode 98 dissolves lithium-air reaction products producedtherein as the cell is discharged. Oxygen depleted air 99 exits thereactor chamber through port 100. Air 84 enters the cell at port 91 andpasses through heat exchanger 90, heat exchanger 105 and heat exchanger92 prior to entering reaction chamber 68. The flow rate can becontrolled by valve 108. The heat exchangers preheat air 84 to a levelsuch that it enters nozzle 87 near the temperature of molten saltelectrolyte 78 exiting nozzle 80. Air entering the reaction chamber 68is heated within heat exchangers 90 and 92 by oxygen depleted air 99exiting the reaction chamber through conduit 88. Air passing throughheat exchanger 105 inside reactor 68 is heated by molten electrolytesalt 78. Extraction of heat from electrolyte 78 in the electrolytereservoir maintains its temperature below the temperature of theelectrolyte 102 that is washing through cathode 98. Electric heater 96is thermally coupled to separator 71 and supplies energy as needed tomaintain the temperature of cathode 98 above the temperature thereservoir electrolyte 78 that is thermally coupled to heat exchanger105. The effect of the thus maintained temperature difference is thatelectrolyte 102 washing through cathode 98 is raised to a highertemperature than electrolyte 78 that is in the reservoir. Continuousflow of electrolyte continuously dissolves and washes away reactionproduct being produced in cathode 98. On the other hand, when theelectrolyte leaves cathode 98 and is cooled by heat exchanger 105 in thereservoir, its saturation limit for dissolved reaction productdecreases, which causes a portion of the reaction product toprecipitate, 97. The electric heater 94 is used to control thetemperature of the electrolyte. The discharge process continues as pump75 resupplies electrolyte 78, now depleted of reaction product, tonozzle 80 where it entrains more air and carries it to cathode 98, isreheated, and dissolves more reaction product as it occurs from lithiumair reactions ongoing therein.

FIG. 6 illustrates operation of the cell under recharge conditions.Power is supplied to heater 94 to increase the solubility level ofreaction product 107 in electrolyte 78. The dissolving of reactionproduct 107 in electrolyte 78 increases with temperature. Pump 75 pumpselectrolyte 78 containing dissolved reaction product to nozzle 80whereby it is sprayed 114 onto cathode 98. Power is applied to terminals82 to electrolyze lithium/air reaction product in cathode 98. With theextraction of electrons 59 by a positive voltage applied to terminal 69relative to terminal 70, reaction product is electrolyzed with oxygen110 being released to escape reactor chamber 68 via port 100. It exitsthe cell through port 78 after passing through heat exchanger 92 and 90to preheat incoming air. During the recharge process, lithium ions areconducted through solid electrolyte separator 71 into reservoir 62 whereit is reduced to lithium by electron flow via terminal 70. The rechargeprocess continuously electrolyzes dissolved reaction product from moltensalt in cathode 98 as reaction product depleted electrolyte 112 returnsto reservoir 78, dissolves more reaction product,107, and is pumped backto cathode 98. Molten lithium is re-supplied to reservoir 62 asindicated by arrow 103. Under recharge condition, valve 108 mayoptionally be closed since air intake into the reaction chamber is notneeded.

In an exemplary cell shown in FIG. 7, solid electrolyte cylinder 2 withterminals 122 and 19 has an inner diameter of 2.54 cm and length of 50cm. The volume of lithium would be 0.253 L (π(2.54(D)/2)²*50cm(L)=253.35 cm³). The electrochemical potential for the lithium/oxygenreaction is 3.14V. Assuming an under load operating output voltage of2.5V to allow for internal impedances, the energy capacity can bedetermined considering the Amp-Hour capacity of lithium being 3,860Ah/kg (2,084 Ah/ltr). At an output voltage of 2.5V, the energy availablefrom the cell would be 9650 Wh/kg (5210 Wh/ltr). Given the 0.253 Llithium volume in the example, the cell could supply 1.3 kWh of energy.

In a cell operating at 300° C. with NaNO₃—KNO₃ molten salt eutecticelectrolyte, the conductivity of the electrolyte is 0.66 S/cm.Similarly, the conductivity of the solid electrolyte containmentcylinder 2 at 300° C. is 0.1 S/cm as shown in FIG. 7. Assuming that thethickness 74 in FIG. 7 of the porous cathode 8 on the surface of thesolid cylinder electrolyte 2 is 0.2 cm and that the thickness 72 of thesolid electrolyte is 0.1 mm, the area specific resistance of the solidelectrolyte plus the liquid can be calculated as 0.403 Ohm-cm² (1/(0.66S/cm)*0.2 cm+1/(0.1 S/cm)*0.01 cm). Given the 0.7 Volt allowance forinternal IR loss, the net output current under load would be 1.73 Aassuming other polarization losses are negligible. In such a case, thearea specific power of the cell would be 4.34 Watts. This example cellhas a surface area of 399 cm²(π*2.54*50), therefor its power outputcapability would be 1.73 kW.

FIG. 8 is an Arrhenius plot showing the conductivity of several solidstate ionic conductive materials that would be suitable for use as theelectrolyte cylinder 2. Impedance line 83 is for lithium beta alumina(data from J. L. Briant, J. Electrochem. Soc.: Electrochemical ScienceAnd Technology; 1834 (1981)) and line 84 is for lithium phosphate glass(data from B. Wang, Journal of Non-Crystalline Solids, Volume 183, Issue3, 2; 297-306 (1995). Conductivity values 82 for aluminum oxide dopedlithium lanthanum zirconium oxide (Al₂O₃:Li₇La₃Zr₂O₁₂) are from MKotobuki, et. al.; Journal of Power Sources 196 7750-7754 (2011)).

Sintered LLZO electrolyte had been demonstrated to be stable withlithium in all solid state batteries. (See T. Yoshida, et. al.; Journalof The Electrochemical Society, 157-10, A1076-A1079 (2010)). The cyclicvoltammogram of the Li/LLZO/Li cell showed that the dissolution anddeposition reactions of lithium occurred reversibly without any reactionwith LLZO. This indicates that a Li metal anode can be employed incontact with LLZO electrolyte.

In an exemplary embodiment, a 1 kWh battery is designed to operate at adischarge rate of 1 C, i.e. battery totally discharged in 1 hour.Lithium has a specific energy of 11,580 Wh/kg. If the mass of the oxygenis included, the net energy density is 5,200 Wh/kg. For a 1 kWh battery,86 g of lithium would be needed. Lithium has a discharge currentcapacity of 3.86 Ah/g. At a discharge rate of 1 C, the requireddischarge current would be 332 A (86 g*3.86 Ah/g/1 hr). In this example,the area of the separator may be defined as 100 cm² and the solidseparator as LLZO or other suitable substitute thereof. In this examplethe use of a 100 cm² separator results in a net current density of 3.32A/cm². As indicated in FIG. 8, the lithium ion conductivity, σ, of LLZOis approximately 0.1 S/cm. A separator made of this material and at athickness, t, of 100 um would have an impedance of 0.1 Ohm-cm², (1/σ*t).The output current supplied at 1 C would have a maximum drop in voltageof 0.4V relative to the cell's open circuit voltage. The primaryreaction product of the cell is Li₂O₂. The amount of air flow requiredto sustain a 1 C discharge rate can be determined from the requiredoxygen flow.

The atomic mass of lithium is 6.9 g/mole. The primary discharge reactionfor the cell is 2Li+O₂>Li₂O₂, 1 mole of oxygen is required for per moleof lithium. The number of moles of lithium in the reaction is 12.46, (86g/6.9 g/mole). Therefore, 6.23 moles or 199.4 grams (6.23 moles *32grams/mole) of oxygen are required to balance the reaction. Air is 23%oxygen by mass so that the total amount of air needed for the reactionis 866 g, (199.4 g O₂/(0.23 g O₂/gAir). For the 1 C discharge, the airmass flow rate is 866 g/hr or 0.24 g/sec. The density of air is 0.00123g/cm³. This gives a volumetric flow rate of 195 cm³/sec.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A rechargeable lithium air battery comprising a ceramicseparator forming an anode chamber, a molten lithium anode contained inthe anode chamber, an air cathode, and a non-aqueous electrolyte,wherein the cathode has a temperature gradient comprising a lowtemperature region and a high temperature region, and wherein thetemperature gradient provides a flow system for reaction productproduced by the battery.
 2. The battery according to claim 1, furthercomprising a heater in the anode chamber.
 3. The battery according toclaim 1, further comprising an electrolyte reservoir adjacent to thecathode.
 4. The battery according to claim 3, further comprising a pumpand a temperature control system.
 5. The battery according to claim 4,wherein the pump controls movement of the electrolyte between thecathode and the electrolyte reservoir.
 6. The battery according to claim4, wherein the temperature control system controls temperatures of thecathode and the electrolyte reservoir.
 7. The battery according to claim1, wherein the cathode comprises a core adjacent to the ceramicseparator and at least one fin extending radially outward from the core,and wherein the core is the high temperature region of the cathode andthe at least one fin is the low temperature region of the cathode. 8.The battery according to claim 1, wherein during discharge the reactionproduct moves from the high temperature region of the cathode to the lowtemperature region of the cathode.
 9. The battery according to claim 1,wherein the electrolyte comprises a molten inorganic salt.
 10. Thebatter according to clam 1, wherein the electrolyte comprises a silaneor siloxane compound.
 11. The battery according to claim 1, wherein thecathode comprises a porous ceramic material.
 12. The battery accordingto claim 11, wherein the cathode is impregnated with a metal nitride ora carbon material.
 13. The battery according to claim 1, wherein thecathode comprises an electrically conductive sintered metal oxide, metalnitride, carbon, or silicon carbide.
 14. The battery according to claim1, wherein the cathode comprises carbon, a polymer binder, and a metaloxide.
 15. The battery according to claim 11, wherein the porous ceramicmaterial comprises lithium lanthanum zirconium oxide.
 16. The batteryaccording to claim 1, where the anode chamber is maintained at about 20°C. to 200° C.
 17. The battery according to claim 1, wherein the ceramicseparator comprises a lithium ion conducting glass.
 18. The batteryaccording to claim 17, wherein the lithium ion conducting glass isselected from lithium beta alumina, lithium phosphate glass, lithiumlanthanum zirconium oxide, Al₂O₃:Li₇La₃Zr₂O₁₂, lithium aluminumgermanium phosphate, and lithium aluminum titanium phosphate.
 19. Thebattery according to claim 6, wherein the temperature of the electrolytereservoir is about 200° C. to about 450° C.
 20. The battery according toclaim 1, wherein the battery has an operating temperature of about 200°C. to about 450° C.